Method and apparatus for random access in wireless communication system and user terminal

ABSTRACT

The present disclosure provides a method and an apparatus for random access in a wireless communication system, as well as a user terminal. The method includes: receiving indication information transmitted from a radio base station, the indication information instructing a user terminal to initiate a random access; determining a target Physical Random Access Channel (PRACH) resource from available resources based on a current coverage level and resource information carried in the indication information, the target PRACH resource being used for transmitting a PRACH signal for initiating the random access; and re-determining, in response to a failure to initiate the random access over the target PRACH resource, the target PRACH resource from the available resources based on an updated coverage level and the resource information, and transmitting the PRACH signal over the re-determined target PRACH resource. The present disclosure can solve the technical problem in the related art associated with low success rate of random accesses initiated by radio base stations.

CROSS REFERENCE TO RELATED APPLICATIONS

This patent application is a continuation of and claims priority to U.S.patent application Ser. No. 16/274,204, filed on Feb. 12, 2019, which isa continuation of and claims priority to International PatentApplication No. PCT/CN2017/078184, filed on Mar. 24, 2017, which claimsthe benefit of priority of Chinese Patent Application No.201610667989.0, filed on Aug. 12, 2016. The entire contents of thebefore-mentioned patent applications are incorporated by reference aspart of the disclosure of this application.

TECHNICAL FIELD

The present disclosure relates to communication technology, and moreparticularly, to a method and an apparatus for random access in awireless communication system, and to a user terminal.

BACKGROUND

In wireless communication systems such as enhanced Machine TypeCommunication (eMTC) and Narrow Band Internet of Things (NB-IoT)standard protocols, a base station (e.g., an evolved NodeB (eNodeB)) caninitiate a random access process e.g., by transmitting a PhysicalDownlink Control Channel (PDCCH) order instructing a User Equipment (UE)to initiate a random access process. In this case, the base station canindicate information on Physical Random Access Channel (PRACH) resourcesto be used by the UE (the PRACH information indicated by the PDCCH orderis shown in Table 1). Upon receiving the instruction from the basestation, the UE transmits PRACH over the resources indicated by the basestation. In the eMTC system, the UE transmits PRACH using a preamblecodeword indicated by the base station.

TABLE 1 eMTC NB-IoT Description Starting CE Starting number Coveragelevel at which UE transmits Level of NPRACH PRACH initially, i.e., UEinitiates repetitions random access over a PRACH resource setcorresponding to the coverage level initially PRACH SubcarrierTime-frequency resource index for Mask indication of UE to transmitPRACH (eMTC) Index NPRACH Frequency-domain sub-carrier ID for UE totransmit PRACH (NB-IoT) Preamble Preamble codeword to be used by IndexUE for PRACH

In the eMTC and NB-IoT systems, PRACH resources are configured dependingon coverage levels. Each coverage level corresponds to a set of PRACHtime-frequency resources, a number of PRACH repetitions and a number ofrandom accesses. In the eMTC system, a set of preamble codewords is alsoconfigured for each coverage level.

For a base station initiated random access process, the PRACH resourceindex, sub-carrier ID or preamble codeword indicated by the base stationis necessarily included in the set of PRACH resources for the initialcoverage level, so as to ensure that the UE can initiate a random accessprocess over the PRACH resources indicated by the base station at theinitial coverage level. However, when the coverage level needs to beincreased after the UE has failed in a number of random accesses overthe PRACH resources indicated by the base station, the PRACH resourceindex, sub-carrier ID or preamble codeword indicated by the base stationmay not be included in the set of PRACH resources for the higher level.In this case, the UE would not be able to initiate a random access orthe UE transmits PRACH according to the PRACH information indicated bythe base station but the base station would not respond. In either case,the random access will fail, resulting in a reduced success rate of thebase station-initiated random accesses.

There is currently no effective solution to the technical problem in therelated art associated with low success rate of random accessesinitiated by radio base stations.

SUMMARY

The present disclosure provides a method and an apparatus for randomaccess in a wireless communication system, as well as a user terminal,capable of at least solving the technical problem in the related artassociated with low success rate of random accesses initiated by radiobase stations.

According to an aspect of the present disclosure, a method for randomaccess in a wireless communication system is provided. The methodincludes: receiving indication information transmitted from a radio basestation, the indication information instructing a user terminal toinitiate a random access; determining a target Physical Random AccessChannel (PRACH) resource from available resources based on a currentcoverage level and resource information carried in the indicationinformation, the target PRACH resource being used for transmitting aPRACH signal for initiating the random access, the current coveragelevel being a coverage level at which the random access is to beinitiated currently, and the available resources being resourcesallowable to be used at the current coverage level; and re-determining,in response to a failure to initiate the random access over the targetPRACH resource, the target PRACH resource from the available resourcesbased on an updated coverage level and the resource information, andtransmitting the PRACH signal over the re-determined target PRACHresource.

Optionally, the failure to initiate the random access over the targetPRACH resource is determined by: obtaining a number of failures toinitiate the random access; and determining the failure to initiate therandom access over the target PRACH resource when the number of failuresreaches a predetermined value that is a maximum number of transmissionsof the PRACH signal allowable at the current coverage level.

Optionally, the method further includes: retransmitting, after obtainingthe number of failures to initiate the random access, the PRACH signalover the target PRACH resource when the number of failures is smallerthan the predetermined value.

Optionally, the updated coverage level is one level higher than thecoverage level before updating.

Optionally, the target PRACH resource includes a target time-frequencyresource, a target codeword and a target sub-carrier for transmittingthe PRACH signal. The operation of determining the target PRACH resourcefrom the available resources based on the current coverage level and theresource information carried in the indication information includes:mapping the resource information to the target PRACH resource fromresources available at the current coverage level.

Optionally, when the wireless communication system is an enhancedMachine Type Communication (eMTC) system, the operation of mapping theresource information to the target PRACH resource from resourcesavailable at the current coverage level includes: determining the targettime-frequency resource based on a number of time-frequency resourcesavailable at the current coverage level and a time-frequency resourceindex in the resource information; and/or determining the targetcodeword based on a start codeword and an end codeword of preambles atthe current coverage level and a preamble index in the resourceinformation.

Optionally, the operation of determining the target time-frequencyresource based on the number of time-frequency resources available atthe current coverage level and the time-frequency resource index in theresource information includes: calculating a first index, ID2, based onthe number, Num1, of the available time-frequency resources and thetime-frequency resource index, ID1, in the resource informationaccording to a predetermined equation of ID2=(ID1−K1)mod(Num1)+K2 orID2=(ID1)mod(Num1)+K3, where K1, K2 and K3 are constants; anddetermining a time-frequency resource corresponding to ID2 as the targettime-frequency resource.

Optionally, the operation of determining the target codeword based onthe start codeword and the end codeword of the preambles at the currentcoverage level and the preamble index in the resource informationincludes: calculating a second index, ID6, based on an index, ID3, ofthe start codeword, an index, ID4, of the end codeword and the preambleindex, ID5, as ID6=ID3+(ID5)mod(ID4−ID3+1), or calculating the secondindex ID6 based on the index ID3 of the start codeword, the index ID4 ofthe end codeword and a start codeword index, ID9, corresponding to acoverage level indicated in the indication information, asID6=ID3+(ID5−ID9)mod(ID4−ID3+1); and determining a codewordcorresponding to ID6 as the target codeword.

Optionally, when the wireless communication system is a Narrow BandInternet of Things (NB-IoT) system, the operation of mapping theresource information to the target PRACH resource from resourcesavailable at the current coverage level includes: determining the targetsub-carrier based on a number of sub-carriers and an index of a startsub-carrier at the current coverage level and a sub-carrier index oroffset in the resource information.

Optionally, the operation of determining the target sub-carrier based onthe number of sub-carriers and the index of the start sub-carrier at thecurrent coverage level and the sub-carrier index or offset in theresource information includes: calculating a third index, ID8, based onthe number, Num2, of sub-carriers and the index, ID7, of the startsub-carrier at the current coverage level and the sub-carrier offset,Num3, in the resource information, as ID8=ID7+(Num3)mod(Num2), orcalculating the third index, ID8, based on the number Num2 ofsub-carriers and the index ID7 of the start sub-carrier at the currentcoverage level, the sub-carrier index Num3 in the resource informationand a start sub-carrier index, ID10, corresponding to a coverage levelindicated in the indication information, asID8=ID7+(Num3−ID10)mod(Num2); and determining the sub-carriercorresponding to ID8 as the target sub-carrier.

Optionally, when the wireless communication system is an eMTC system oran NB-IoT system, the indication information is PDCCH order information.

Optionally, when the wireless communication system is an eMTC system,the indication information is handover information.

According to another aspect of the present disclosure, an apparatus forrandom access in a wireless communication system is provided. Theapparatus includes: a receiving unit configured to receive indicationinformation transmitted from a radio base station, the indicationinformation instructing a user terminal to initiate a random access; adetermining unit configured to determine a target Physical Random AccessChannel (PRACH) resource from available resources based on a currentcoverage level and resource information carried in the indicationinformation, the target PRACH resource being used for transmitting aPRACH signal for initiating the random access, and the current coveragelevel being a coverage level at which the random access is to beinitiated currently; and a transmitting unit configured to re-determine,in response to a failure to initiate the random access over the targetPRACH resource, the target PRACH resource from the available resourcesbased on an updated coverage level and the resource information, andtransmit the PRACH signal over the re-determined target PRACH resource.

Optionally, the transmitting unit includes: an obtaining moduleconfigured to obtain a number of failures to initiate the random access;and a first determining module configured to determine the failure toinitiate the random access over the target PRACH resource when thenumber of failures reaches a predetermined value that is a maximumnumber of transmissions of the PRACH signal allowable at the currentcoverage level.

Optionally, the transmitting unit further includes: a transmittingmodule configured to retransmit, after obtaining the number of failuresto initiate the random access, the PRACH signal over the target PRACHresource when the number of failures is smaller than the predeterminedvalue.

Optionally, the updated coverage level is one level higher than thecoverage level before updating.

Optionally, the target PRACH resource includes a target time-frequencyresource, a target codeword and a target sub-carrier for transmittingthe PRACH signal. The determining unit is further configured to map theresource information to the target PRACH resource from resourcesavailable at the current coverage level.

Optionally, when the wireless communication system is an enhancedMachine Type Communication (eMTC) system, the determining unit includes:a second determining module configured to determine the targettime-frequency resource based on a number of time-frequency resourcesavailable at the current coverage level and a time-frequency resourceindex in the resource information; and/or a third determining moduleconfigured to determine the target codeword based on a start codewordand an end codeword of preambles at the current coverage level and apreamble index in the resource information.

Optionally, the second determining module is further configured to:calculate a first index, ID2, based on the number, Num1, of theavailable time-frequency resources and the time-frequency resourceindex, ID1, in the resource information according to a predeterminedequation of ID2=(ID1−K1)mod(Num1)+K2 or ID2=(ID1)mod(Num1)+K3, where K1,K2 and K3 are constants; and determine a time-frequency resourcecorresponding to ID2 as the target time-frequency resource.

Optionally, the third determining module is further configured to:calculate a second index, ID6, based on an index, ID3, of the startcodeword, an index, ID4, of the end codeword and the preamble index,ID5, as ID6=ID3+(ID5)mod(ID4−ID3+1), or calculate the second index ID6based on the index ID3 of the start codeword, the index ID4 of the endcodeword and a start codeword index, ID9, corresponding to a coveragelevel indicated in the indication information, asID6=ID3+(ID5−ID9)mod(ID4−ID3+1); and determine a codeword correspondingto ID6 as the target codeword.

Optionally, when the wireless communication system is a Narrow BandInternet of Things (NB-IoT) system, the determining unit includes: afourth determining unit configured to determine the target sub-carrierbased on a number of sub-carriers and an index of a start sub-carrier atthe current coverage level and a sub-carrier index or offset in theresource information.

Optionally, the fourth determining module is further configured to:calculate a third index, ID8, based on the number, Num2, of sub-carriersand the index, ID7, of the start sub-carrier at the current coveragelevel and the sub-carrier offset, Num3, in the resource information, asID8=ID7+(Num3)mod(Num2), or calculate the third index, ID8, based on thenumber Num2 of sub-carriers and the index ID7 of the start sub-carrierat the current coverage level, the sub-carrier index Num3 in theresource information and a start sub-carrier index, ID10, correspondingto a coverage level indicated in the indication information, asID8=ID7+(Num3−ID10)mod(Num2); and determine the sub-carriercorresponding to ID8 as the target sub-carrier.

Optionally, when the wireless communication system is an eMTC system oran NB-IoT system, the indication information is PDCCH order information.

Optionally, when the wireless communication system is an eMTC system,the indication information is handover information.

According to a further aspect of the present disclosure, a user terminalis provided. The user terminal includes a memory storing codes for stepsof: receiving indication information transmitted from a radio basestation, the indication information instructing a user terminal toinitiate a random access; determining a target Physical Random AccessChannel (PRACH) resource from available resources based on a currentcoverage level and resource information carried in the indicationinformation, the target PRACH resource being used for transmitting aPRACH signal for initiating the random access; and re-determining, inresponse to a failure to initiate the random access over the targetPRACH resource, the target PRACH resource from the available resourcesbased on an updated coverage level and the resource information, andtransmitting the PRACH signal over the re-determined target PRACHresource. The user terminal further includes a processor for executingthe codes stored in the memory. The user terminal further includes atransmission device connected to the memory and the processor, andconfigured to transmit the codes in the memory to the processor.

According to yet a further aspect of the present disclosure, a storagemedium is provided. The storage medium can be configured to store codesfor steps of: receiving indication information transmitted from a radiobase station, the indication information instructing a user terminal toinitiate a random access; determining a target Physical Random AccessChannel (PRACH) resource from available resources based on a currentcoverage level and resource information carried in the indicationinformation, the target PRACH resource being used for transmitting aPRACH signal for initiating the random access; and re-determining, inresponse to a failure to initiate the random access over the targetPRACH resource, the target PRACH resource from the available resourcesbased on an updated coverage level and the resource information, andtransmitting the PRACH signal over the re-determined target PRACHresource.

In the present disclosure, indication information transmitted from aradio base station is received. The indication information instructs auser terminal to initiate a random access. A target Physical RandomAccess Channel (PRACH) resource is determined from available resourcesbased on a current coverage level and resource information carried inthe indication information. The target PRACH resource is used fortransmitting a PRACH signal for initiating the random access. Inresponse to a failure to initiate the random access over the targetPRACH resource, the target PRACH resource is re-determined from theavailable resources based on an updated coverage level and the resourceinformation, and the PRACH signal is transmitted over the re-determinedtarget PRACH resource. In this way, it is possible to solve thetechnical problem in the related art associated with low success rate ofrandom accesses initiated by radio base stations, thereby achieving atechnical effect of an increased success rate of random accesses.

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure can be further understood with reference to thefigures described below, which constitute a part of the presentdisclosure. The illustrative embodiments of the present disclosure anddescriptions thereof are provided for explaining, rather than limiting,the present disclosure. In the figures:

FIG. 1 is a schematic diagram showing an optional user terminalaccording to the present disclosure;

FIG. 2 is a flowchart illustrating a process of random access in awireless communication system according to the present disclosure;

FIG. 3 is a flowchart illustrating an alternative process of randomaccess in a wireless communication system according to the presentdisclosure

FIG. 4 is a flowchart illustrating an alternative process of randomaccess in a wireless communication system according to the presentdisclosure; and

FIG. 5 is a schematic diagram showing an apparatus for random access ina wireless communication system according to the present disclosure.

DETAILED DESCRIPTION OF THE EMBODIMENTS

In the following, the present disclosure will be described in detailwith reference to the figures, taken in conjunction with theembodiments. The embodiments, and the features thereof, can be combinedwith each other, provided that they do not conflict.

It is to be noted that, the terms such as “first”, “second” and so on inthe description, claims and figures are used for distinguishing amongsimilar objects and do not necessarily imply any particularly order orsequence.

Embodiment 1

The method provided according to Embodiment 1 of the present disclosurecan be performed in a user terminal, a computer terminal or a similarcomputing device. When the method is performed in a user terminal forexample, as shown in FIG. 1 , the user terminal can include: one or moreprocessors 101 (only one is shown, which includes, but not limited to, aprocessing device like a microprocessor such as MCU or a programmablelogic device such as FPGA), a memory 103 for storing data, and atransmission device 105 for providing communication functions. It can beappreciated by those skilled in the art that the structure shown in FIG.1 is illustrative only, and the structure of the above electronic deviceis not limited thereto.

The memory 103 can store software programs and modules of softwareapplications, e.g., program instructions/modules associated with themethod for device control according to the present disclosure. Theprocessor 101 performs various functional applications and dataprocessing operations, i.e., performing the above method, by executingthe software programs and modules stored in the memory 103. The memorymay include a random cache or non-volatile memory such as one or moremagnetic storage devices, flash memories or other non-volatilesolid-state memories. In some examples, the memory may further includeone or more memories which are remote from the processor and can beconnected to the computer terminal via a network. Examples of suchnetwork include, but not limited to, Internet, an intranet of anenterprise, a Local Area Network (LAN), a mobile communication network,and any combination thereof.

It is to be noted that the memory can store codes for steps of:receiving indication information transmitted from a radio base station,the indication information instructing a user terminal to initiate arandom access; determining a target Physical Random Access Channel(PRACH) resource from available resources based on a current coveragelevel and resource information carried in the indication information,the target PRACH resource being used for transmitting a PRACH signal forinitiating the random access and the current coverage level being acoverage level at which the random access is to be initiated currently;and re-determining, in response to a failure to initiate the randomaccess over the target PRACH resource, the target PRACH resource fromthe available resources based on an updated coverage level and theresource information, and transmitting the PRACH signal over there-determined target PRACH resource.

The transmission device can transmit or receive data via a network. Thenetwork can be e.g., a wireless network provided by a communicationprovider of the computer terminal. In an example, the transmissiondevice includes a network adaptor, or Network Interface Controller(NIC), which can be connected to other network devices via a basestation for communication with Internet. In an example, the transmissiondevice can be a Radio Frequency (RF) module for communicating withInternet wirelessly.

According to an embodiment of the present disclosure, a method forrandom access in a wireless communication system is provided. It is tobe noted that, the steps shown in the flowchart can be performed in acomputer system executing a set of computer executable instructions.While a logical sequence order is shown in the flowchart, in some casesthe steps as shown or described can be performed in a different orderthan the one as shown herein.

FIG. 2 is a flowchart illustrating a process of random access in awireless communication system according to the present disclosure. Asshown in FIG. 2 , the process flow includes the following steps.

At step S201, indication information transmitted from a radio basestation is received. The indication information instructs a userterminal to initiate a random access.

At step S202, a target PRACH resource is determined from availableresources based on a current coverage level and resource informationcarried in the indication information. The target PRACH resource is usedfor transmitting a PRACH signal for initiating the random access. Thecurrent coverage level is a coverage level at which the random access isto be initiated currently. The available resources are resourcesallowable to be used at the current coverage level.

At step S203, in response to a failure to initiate the random accessover the target PRACH resource, the target PRACH resource isre-determined from the available resources based on an updated coveragelevel and the resource information, and the PRACH signal is transmittedover the re-determined target PRACH resource.

With the above embodiments, indication information transmitted from aradio base station is received. The indication information instructs auser terminal to initiate a random access. A target PRACH resource isdetermined from available resources based on a current coverage leveland resource information carried in the indication information. Thetarget PRACH resource is used for transmitting a PRACH signal forinitiating the random access. In response to a failure to initiate therandom access over the target PRACH resource, the target PRACH resourceis re-determined from the available resources based on an updatedcoverage level and the resource information, and the PRACH signal istransmitted over the re-determined target PRACH resource. In this way,it is possible to solve the technical problem in the related artassociated with low success rate of random accesses initiated by radiobase stations, thereby achieving a technical effect of an increasedsuccess rate of random accesses.

Optionally, the above steps may be performed by a user terminal, as anon-limiting example.

At the step S201, when the indication information transmitted from theradio base station is received, the indication information may includePDCCH order information. That is, a base station, or base station, maytransmit the indication information via a PDCCH order. However, thepresent disclosure it not limited to this, and other types of basestations may use similar messages for transmission.

At the step S202, when the target PRACH resource is determined fromavailable resources based on the current coverage level and the resourceinformation carried in the indication information, the resourceinformation may be mapped to the target PRACH resource from resourcesavailable at the current coverage level. The target PRACH resource maybe determined based on a scheme corresponding to a system type of theradio base station.

When the wireless communication system is an eMTC system, the targetPRACH resource can include a target time-frequency resource and a targetcodeword for transmitting the PRACH signal. The operation of determininga target PRACH resource from available resources based on a currentcoverage level and resource information carried in the indicationinformation can include: determining the target time-frequency resourcebased on a number of time-frequency resources available at the currentcoverage level and a time-frequency resource index in the resourceinformation; and/or determining the target codeword based on a startcodeword and an end codeword of preambles at the current coverage leveland a preamble index in the resource information.

Optionally, the operation of determining the target time-frequencyresource based on the number of time-frequency resources available atthe current coverage level and the time-frequency resource index in theresource information can include: calculating a first index, ID2, basedon the number, Num1, of the available time-frequency resources and thetime-frequency resource index, ID1, in the resource informationaccording to a predetermined equation of ID2=(ID1−K1)mod(Num1)+K2 orID2=(ID1)mod(Num1)+K3, where K1, K2 and K3 are constants; anddetermining a time-frequency resource corresponding to ID2 as the targettime-frequency resource.

The above “mod” represents a modulo operation.

The operation of determining the target codeword based on the startcodeword and the end codeword of the preambles at the current coveragelevel and the preamble index in the resource information can include:calculating a second index, ID6, based on an index, ID3, of the startcodeword, an index, ID4, of the end codeword and the preamble index,ID5, as ID6=ID3+(ID5)mod(ID4−ID3+1), or calculating the second index ID6based on the index ID3 of the start codeword, the index ID4 of the endcodeword and a start codeword index, ID9, corresponding to a coveragelevel indicated in the indication information, asID6=ID3+(ID5−ID9)mod(ID4−ID3+1); and determining a codewordcorresponding to ID6 as the target codeword.

When the wireless communication system is a NB-IoT system, the targetPRACH resource can include a target sub-carrier for transmitting thePRACH signal, and the operation of mapping the resource information tothe target PRACH resource from resources available at the currentcoverage level can include: determining the target sub-carrier based ona number of sub-carriers and an index of a start sub-carrier at thecurrent coverage level and a sub-carrier index or offset in the resourceinformation.

Optionally, the operation determining the target sub-carrier based onthe number of sub-carriers and the index of the start sub-carrier at thecurrent coverage level and the sub-carrier index or offset in theresource information can include: calculating a third index, ID8, basedon the number, Num2, of sub-carriers and the index, ID7, of the startsub-carrier at the current coverage level and the sub-carrier offset,Num3, in the resource information, as ID8=ID7+(Num3)mod(Num2), orcalculating the third index, ID8, based on the number Num2 ofsub-carriers and the index ID7 of the start sub-carrier at the currentcoverage level, the sub-carrier index Num3 in the resource informationand a start sub-carrier index, ID10, corresponding to a coverage levelindicated in the indication information, asID8=ID7+(Num3−ID10)mod(Num2); and determining the sub-carriercorresponding to ID8 as the target sub-carrier.

At the step S203, the failure to initiate the random access over thetarget PRACH resource may be determined by: obtaining a number offailures to initiate the random access; and determining the failure toinitiate the random access over the target PRACH resource when thenumber of failures reaches a predetermined value that is a maximumnumber of transmissions of the PRACH signal allowable at the currentcoverage level.

It is to be noted that after obtaining the number of failures toinitiate the random access, the PRACH signal may be retransmitted overthe target PRACH resource when the number of failures is smaller thanthe predetermined value, until the number of failures reaches thepredetermined value. The updated coverage level is one level higher thanthe coverage level before updating.

In the following, the embodiment of the present disclosure will bedescribed in detail with reference to various implementations shown inFIGS. 3 and 4 .

Implementation 1

It is to be noted that an eMTC FDD system is configured with fourcoverage levels, CEL0, CEL1, CEL2 and CEL3, among which CEL0 is thelowest coverage level. PRACH resource information for the four coveragelevels is shown in Table 2.

TABLE 2 Threshold PRACH PRACH for no. of PRACH start frequency- randomaccesses Coverage config. subframe domain Preamble (with a total levelindex period offset codeword number of 20) CEL0 12  2 0  0-11 3 CEL1  9 4 0 12-23 3 CEL2  6  8 0 24-35 3 CEL3  3 16 0 36-37 3

According to the 36.211 specification, the PRACH time-domain resources(radio frame numbers and subframe numbers) corresponding to the abovePRACH configuration indices are shown in Table 3.

TABLE 3 PRACH config. index Radio frame No. Subframe No. 3 Any 1 6 Any1, 6 9 Any 1, 4, 7 12  Any 0, 2, 4, 6, 8

At step S301, in an eMTC system, an base station instructs a UE toinitiate a random access (e.g., by transmitting a PDCCH order). Theindicated PRACH resource information includes an initial coverage leveland a subcarrier indication of NPRACH (i.e., subcarrier index). In thePDCCH order transmitted from the base station, the indicated PRACHinformation is shown in Table 4.

TABLE 4 eMTC Description Value Starting CE Coverage level at which UEtransmits CEL1 Level PRACH initially, i.e., UE initiates random accessover a PRACH resource set corresponding to the coverage level initiallyPRACH Mask Time-frequency resource index for UE to  3 Index transmitPRACH Preamble Index Preamble code to be used by UE for PRACH 15

At step S302, the UE receives the PDCCH order, and obtains informationon the initial coverage level and obtains a PRACH resource correspondingto the level, including a time-domain location, a frequency-domainlocation and a preamble codeword. The number of available PRACHtime-frequency resources is NPRACH.

For example, the UE obtains time-frequency resource information of aPRACH resource set and a number of repetitions based on the initialcoverage level. The UE can calculate a PRACH sub-carrier ID to be usedbased on the subcarrier indication of NPRACH and the number of PRACHsub-carriers available at the initial coverage level.

At step S303, the UE transmits PRACH over the determined PRACH resource.

If the PRACH Mask Index (i.e., time-frequency resource index)transmitted from the base station is within a range of 1-10, the UE cancalculate/determine a UE Mask Index (i.e., a target time-frequencyresource index) to be used based on the PRACH Mask Index and the numberof PRACH time-frequency resources (NPRACH), by means of mod operation:UE Mask Index=(PRACH Mask Index-1)mod(NPRACH)+1, orUE Mask Index=(PRACH Mask Index)mod(NPRACH)+1.

The number of PRACH time-frequency resources can be obtained from PRACHresource configuration according to the 36.211 specification.

The definitions according to the 36.321 specification apply when thePRACH Mask Index field has other values.

The UE can calculate the Preamble ID (i.e., target sub-carrier index) tobe used based on the Preamble Index (code index) transmitted from thebase station and the number of preamble codewords (NPreamble):Preamble ID=(firstPreamble-r13)+(Preamble Index)mod(NPreamble),NPreamble=(lastPreamble-r13)−(firstPreamble-r13)+1,

where “firstPreamble-r13” is an element defined in the 36.331specification, which indicates a start preamble codeword at the currentcoverage level, and “lastPreamble-r13” is an element defined in the36.331 specification, which indicates the last preamble codeword usedfor the current coverage level.

The UE transmits the PRACH based on the PRACH and preamble code asdetermined above.

At step S304, it is determined whether the random access has succeeded.If so, the method proceeds with step S306; otherwise the method proceedswith step S305.

At step S305, if the initiated random access process has failed, PRACHis retransmitted over the determined PRACH resource. It is determinedwhether the number of PRACH transmissions at the initial coverage levelis smaller than a maximum number of random accesses. If so, the methodproceeds with step S307; otherwise the method proceeds with step S306.

For example, the UE receives the PDCCH order, obtains the initialcoverage level of CEL1 and the PRACH resource corresponding to CEL1. ThePRACH configuration index is 9, i.e., the preamble code is to betransmitted in Subframes 1, 4 and 7 in each radio frame. The number,NPRACH, of PRACH resources available in this configuration is 3. Thestart subframe period for PRACH is 4 subframes. The start PRB of PRACHin the frequency domain is PRB=0. The number of preamble codes is 12,ranging from 12 to 23. The number of random accesses initiated at CEL1is min(3, 20)=3.

The UE calculates the UE Mask Index to be used based on the PRACH MaskIndex indicated in the PDCCH order: UE Mask Index=(PRACH Mask Index-1)mod NPRACH+1=(3−1) mod 3+1=3. According to the 36.321 and 36.211specifications, the preamble code is transmitted in Subframe 7 of eachradio frame. Since the start subframe period for PRACH is 4 subframes,the UE transmits the preamble code in the subframe satisfying

${{\left( {{sf} + \left\lfloor \frac{ns}{2} \right\rfloor} \right)\mspace{14mu}{mod}\mspace{14mu} 40} = 7},$where sf is a radio frame number and ns is a slot number. The UEcalculates the UE Mask Index to be used based on the Preamble Indexindicated in the PDCCH order: UE Mask Index=12+15 mod 12=15. The UEtransmits the preamble code 15 over the PRACH resource having PRB=0 inthe determined subframe. If the initiated random access process failsand the number of random accesses initiated at CEL0 is smaller than 3,the preamble code 15 is retransmitted over the PRACH resource havingPRB=0 in the subframe satisfying

${\left( {{sf} + \left\lfloor \frac{ns}{2} \right\rfloor} \right)\mspace{14mu}{mod}\mspace{14mu} 40} = 7.$

At step S306, the random access process ends.

At step S307, it is determined whether the number of PRACH transmissionsat the initial coverage level is smaller than the number of randomaccesses for this coverage level. If so, the method proceeds with stepS303; otherwise the method proceeds with step S308.

At step 308, if the number of PRACH transmissions at the currentcoverage level is equal to max(NumPreambleAttemptCE-r13) (i.e., thenumber of random accesses for the current coverage level) and smallerthan num(RepetitionsPerPreambleAttempt-r13) (i.e., the maximum number ofrandom accesses), the coverage level is increased by one level and thestep S303 is performed again.

The PRACH resource corresponding to the level is obtained, including atime-domain location, a frequency-domain location and a preamblecodeword.

The UE obtains time-frequency resource information of a PRACH resourceset and a number of repetitions based on the current coverage level. TheUE can calculate a PRACH sub-carrier ID to be used based on thesubcarrier indication of NPRACH and the number of PRACH sub-carriersavailable at the current coverage level.

The UE calculates the PRACH UE Mask Index and Preamble ID to be used atthe new coverage level and transmits PRACH over the determined PRACHresource.

For example, if the number of random accesses initiated at CEL1 is 3(which has not reached the total number threshold of 20), the UEincreases the coverage level to CEL2 and obtains the PRACH resourcecorresponding to the level. The PRACH configuration index is 6, i.e.,the preamble code is to be transmitted in Subframes 1 and 6 in eachradio frame. The number, Nprach, of PRACH resources available in thisconfiguration is 2. The start subframe period for PRACH is 8 subframes.The start PRB of PRACH in the frequency domain is PRB=0. The number ofpreamble codes is 12, ranging from 24 to 35. The number of randomaccesses initiated at CEL2 is min(3, 20−3)=3.

The UE calculates the UE Mask Index to be used based on the PRACH MaskIndex indicated in the PDCCH order: UE Mask Index=(3−1) mod 2+1=1.According to the 36.321 and 36.211 specifications, the preamble code istransmitted in Subframe 1 of each radio frame. Since the start subframeperiod for PRACH is 8 subframes, the UE transmits the preamble code inthe subframe satisfying

${{\left( {{sf} + \left\lfloor \frac{ns}{2} \right\rfloor} \right)\mspace{14mu}{mod}\mspace{14mu} 80} = 1},$where sf is a radio frame number and ns is a slot number. The UEcalculates the preamble index (i.e., Preamble ID) to be used based onthe Preamble Index indicated in the PDCCH order: 24+15 mod 12=27. The UEtransmits the preamble code 27 over the PRACH resource having PRB=0 inthe determined subframe. The above steps are repeated until the randomaccess succeeds or until the maximum number of repetitionsmin(3+3+3,20)=9 is reached.

Alternatively, the UE receives the PDCCH order, obtains the initialcoverage level of CEL1 and the PRACH resource corresponding to CEL1. ThePRACH configuration index is 9, i.e., the preamble code is to betransmitted in Subframes 1, 4 and 7 in each radio frame. The number,Nprach, of PRACH resources available in this configuration is 3. Thestart subframe period for PRACH is 4 subframes. The start PRB of PRACHin the frequency domain is PRB=0. The number of preamble codes is 12,ranging from 12 to 23. The number of random accesses initiated at CEL1is min(3, 20)=3. The UE calculates the mask index to be used based onthe PRACH Mask Index indicated in the PDCCH order: mask index=PRACH MaskIndex mod Nprach+1=3 mod 3+1=1. According to the 36.321 and 36.211specifications, mask index=3, i.e., the preamble code is transmitted inSubframe 7 of each radio frame. Since the start subframe period forPRACH is 4 subframes, the UE transmits the preamble code in the subframesatisfying

${{\left( {{sf} + \left\lfloor \frac{ns}{2} \right\rfloor} \right)\mspace{14mu}{mod}\mspace{14mu} 40} = 7},$where sf is a radio frame number and ns is a slot number.

The UE calculates the preamble index to be used based on the PreambleIndex indicated in the PDCCH order: preamble index=12+Preamble Index mod12=12+15 mod 12=15. The UE transmits the preamble code 15 over the PRACHresource having PRB=0 in the determined subframe. If the initiatedrandom access process fails and the number of random accesses initiatedat CEL0 is smaller than 3, the preamble code 15 is retransmitted overthe PRACH resource having PRB=0 in the subframe satisfying

${\left( {{sf} + \left\lfloor \frac{ns}{2} \right\rfloor} \right)\mspace{14mu}{mod}\mspace{14mu} 40} = 7.$

If the number of random accesses initiated at CEL1 is 3 (which has notreached the total number threshold of 20), the UE increases the coveragelevel to CEL2 and obtains the PRACH resource corresponding to the level.The PRACH configuration index is 6, i.e., the preamble code is to betransmitted in Subframes 1 and 6 in each radio frame. The number,NPRACH, of PRACH resources available in this configuration is 2. Thestart subframe period for PRACH is 8 subframes. The start PRB of PRACHin the frequency domain is PRB=0. The number of preamble codes is 12,ranging from 24 to 35. The number of random accesses initiated at CEL2is min(3, 20−3)=3.

The UE calculates the mask index to be used based on the PRACH MaskIndex indicated in the PDCCH order: mask index=(PRACH Mask Index-1) modNPRACH+1=(3−1) mod 2+1=1. According to the 36.321 and 36.211specifications, mask index=1, i.e., the preamble code is transmitted inSubframe 1 of each radio frame.

Since the start subframe period for PRACH is 8 subframes, the UEtransmits the preamble code in the subframe satisfying

${{\left( {{sf} + \left\lfloor \frac{ns}{2} \right\rfloor} \right)\mspace{14mu}{mod}\mspace{14mu} 80} = 1},$where sf is a radio frame number and ns is a slot number.

The UE calculates the preamble index to be used based on the PreambleIndex indicated in the PDCCH order: preamble index=24+Preamble Index mod12=24+15 mod 12=27. The UE transmits the preamble code 27 over the PRACHresource having PRB=0 in the determined subframe. The above steps arerepeated until the random access succeeds or until the maximum number ofrepetitions min(3+3+3,20)=9 is reached.

Implementation 2

It is to be noted that an NB-IoT system is configured with threecoverage levels, CEL0, CEL1 and CEL2, among which CEL0 is the lowestcoverage level. PRACH resource information for the three coverage levelsis shown in Table 5.

TABLE 5 Threshold for no. of random Coverage PRACH PRACH start PRACHsub- accesses (with a level period subframe offset carrier total numberof 20) CEL0 640 ms  8 ms  0-11 3 CEL1 640 ms  64 ms 12-23 3 CEL2 640 ms256 ms 24-35 3

At step S401, in an NB-IoT system, an base station transmits a PDCCHorder indicating PRACH information as shown in Table 6, including a“Starting number of NPRACH repetitions” field and a “Subcarrierindication of NPRACH” field.

TABLE 6 Description Value Starting number of Coverage level at which UEtransmits CEL0 NPRACH PRACH initially, i.e., UE initiates repetitionsrandom access over a PRACH resource set corresponding to the coveragelevel initially Subcarrier Frequency-domain sub-carrier ID (NB- 6indication of IoT) for UE to transmit PRACH NPRACH

At step S402, the UE receives the PDCCH order, and obtains informationon the initial coverage level and obtains a PRACH resource correspondingto the level, including a time-domain location and a frequency-domainlocation. Here, the frequency-domain location can be determined based ontwo parameters: nprach-SubcarrierOffset-r13 (an index of the firstsubcarrier allocated to PRACH) and nprach-NumSubcarriers-r13.

For example, the initial coverage level of CEL0 and the PRACH resourcecorresponding to CEL0 can be obtained. Its period is 640 ms, the startsubframe offset is 8 and the number of sub-carriers is 12, ranging from0 to 11. The number of random accesses initiated at CEL0 is min(3,20)=3. The UE can calculate the sub-carrier ID to be used at CEL0 basedon the indicated Subcarrier indication of NPRACH. The calculation caninclude but not limited to:

PRACH Sub-carrier ID=nprach-SubcarrierOffset-r13+Subcarrier indicationof NPRACH mod nprach-NumSubcarriers-r13=0+6 mod 12=6.

At a step S403, when the end subframe of the PDCCH order is n, the UEtransmits PRACH over the determined PRACH resource, starting from thesubframe having an index of n+9.

For example, the UE transmits a PRACH signal, starting from a subframesatisfying

${{\left( {{sf} + \left\lfloor \frac{ns}{2} \right\rfloor} \right)\mspace{14mu}{mod}\mspace{14mu} 640} = 8},$using a sub-carrier ID of 6, where sf is a radio frame number and ns isa slot number. If the initiated random access process fails and thenumber of random accesses initiated at CEL0 is smaller than 3, the PRACHsignal is retransmitted, starting from a subframe satisfying

${{\left( {{sf} + \left\lfloor \frac{ns}{2} \right\rfloor} \right)\mspace{14mu}{mod}\mspace{14mu} 640} = 8},$using a sub-carrier ID of 6.

At step S404, it is determined whether the random access has succeeded.If so, the method proceeds with step S406; otherwise the method proceedswith step S405.

If the number of random accesses initiated at CEL0 is 3 (which has notreached the total number threshold of 20), the UE increases the coveragelevel to CEL1 and obtains the PRACH resource corresponding to the level.Its period is 640 ms, the start subframe offset is 64 and the number ofsub-carriers is 12, ranging from 12 to 23. The number of random accessesinitiated at CEL1 is min(3, 20-3)=3.

At step S405, if the initiated random access process has failed, PRACHis retransmitted over the determined PRACH resource. It is determinedwhether the number of PRACH transmissions at the initial coverage levelis smaller than a maximum number of random accesses. If so, the methodproceeds with step S407; otherwise the method proceeds with step S406.

At step S406, the random access process ends.

At step S407, it is determined whether the number of PRACH transmissionsat the initial coverage level is smaller than the number of randomaccesses for this coverage level. If so, the method proceeds with stepS403; otherwise the method proceeds with step S408.

At step 408, if the number of PRACH transmissions at the currentcoverage level is equal to max(NumPreambleAttemptCE-r13) (i.e., thenumber of random accesses for the current coverage level) and smallerthan num(RepetitionsPerPreambleAttempt-r13) (i.e., the maximum number ofrandom accesses), the coverage level is increased by one level, thePRACH resource corresponding to the level is obtained, including atime-domain location, a frequency-domain location and a number ofrepetitions, and the step S403 is performed again.

The UE calculates the PRACH sub-carrier ID to be used at the newcoverage level.

The UE transmits PRACH over the re-determined PRACH resource.

For example, the UE can calculate the sub-carrier ID to be used at CEL1based on the indicated Subcarrier indication of NPRACH. The calculationcan include but not limited to: PRACH sub-carrier ID=12+6 mod 12=18. TheUE retransmits the PRACH signal, starting from a subframe satisfying

${{\left( {{sf} + \left\lfloor \frac{ns}{2} \right\rfloor} \right)\mspace{14mu}{mod}\mspace{14mu} 640} = 64},$using a sub-carrier ID of 18.

The above steps are repeated until the random access succeeds or untilthe maximum number of repetitions min(3+3+3,20)=9 is reached.

With the description of the above embodiments, it will be apparent tothose skilled in the art that the method according to the aboveembodiments can be realized by means of software plus the necessarygeneral hardware platform, and of course can be implemented viahardware, but in many cases the former is the better implementation.Based on this understanding, the technical solution of the presentdisclosure, in essence or in terms of parts contributing to the priorart, can be embodied in the form of a software product, which is storedin a storage medium (e.g., ROM/RAM, disc, CD-ROM) and includes a numberof instructions for enabling a terminal device (which may be a mobilephone, a computer, a server, or a network device, etc.) to perform themethod described in the various embodiments of the present disclosure.

Implementation 3

It is to be noted that an NB-IoT system is configured with threecoverage levels, CEL0, CEL1 and CEL2, among which CEL0 is the lowestcoverage level. PRACH resource information for the three coverage levelsis shown in Table 7.

TABLE 7 Threshold for no. of random Coverage PRACH PRACH start PRACHsub- accesses (with a level period subframe offset carrier total numberof 20) CEL0 640 ms  8 ms  0-11 3 CEL1 640 ms  64 ms 12-23 3 CEL2 640 ms256 ms 24-35 3

At step S401, in an NB-IoT system, an base station transmits a PDCCHorder indicating PRACH information as shown in Table 8, including a“Starting number of NPRACH repetitions” field and a “Subcarrierindication of NPRACH” field.

TABLE 8 Description Value Starting number of Coverage level at which UEtransmits CEL0 NPRACH PRACH initially, i.e., UE initiates repetitionsrandom access over a PRACH resource set corresponding to the coveragelevel initially Subcarrier Frequency-domain sub-carrier ID (NB- 6indication of IoT) for UE to transmit PRACH NPRACH

At step S402, the UE receives the PDCCH order, and obtains informationon the initial coverage level and obtains a PRACH resource correspondingto the level, including a time-domain location and a frequency-domainlocation. Here, the frequency-domain location can be determined based ontwo parameters: nprach-SubcarrierOffset-r13 (PRACH start sub-carrier)and nprach-NumSubcarriers-r13.

For example, the initial coverage level of CEL0 and the PRACH resourcecorresponding to CEL0 can be obtained. Its period is 640 ms, the startsubframe offset is 8 and the number of sub-carriers is 12, ranging from0 to 11. The number of random accesses initiated at CEL0 is min(3,20)=3. The UE can calculate the sub-carrier ID to be used at CEL0 basedon the indicated Subcarrier indication of NPRACH. The calculation caninclude but not limited to:

PRACH Sub-carrier ID=nprach-SubcarrierOffset-r13+(Subcarrier indicationof NPRACH-nprach-SubcarrierOffset-r13) modnprach-NumSubcarriers-r13=0+(6-0) mod 12=6.

At a step S403, when the end subframe of the PDCCH order is n, the UEtransmits PRACH over the determined PRACH resource, starting from thesubframe having an index of n+9.

For example, the UE transmits a PRACH signal, starting from a subframesatisfying

${{\left( {{sf} + \left\lfloor \frac{ns}{2} \right\rfloor} \right){mod}\mspace{14mu} 640} = 8},$using a sub-carrier ID of 6, where sf is a radio frame number and ns isa slot number. If the initiated random access process fails and thenumber of random accesses initiated at CEL0 is smaller than 3, the PRACHsignal is retransmitted, starting from a subframe satisfying

${{\left( {{sf} + \left\lfloor \frac{ns}{2} \right\rfloor} \right){mod}\mspace{14mu} 640} = 8},$using a sub-carrier ID of 6.

At step S404, it is determined whether the random access has succeeded.If so, the method proceeds with step S406; otherwise the method proceedswith step S405.

If the number of random accesses initiated at CEL0 is 3 (which has notreached the total number threshold of 20), the UE increases the coveragelevel to CEL1 and obtains the PRACH resource corresponding to the level.Its period is 640 ms, the start subframe offset is 64 and the number ofsub-carriers is 12, ranging from 12 to 23. The number of random accessesinitiated at CEL1 is min(3, 20-3)=3.

At step S405, if the initiated random access process has failed, PRACHis retransmitted over the determined PRACH resource. It is determinedwhether the number of PRACH transmissions at the initial coverage levelis smaller than a maximum number of random accesses. If so, the methodproceeds with step S407; otherwise the method proceeds with step S406.

At step S406, the random access process ends.

At step S407, it is determined whether the number of PRACH transmissionsat the initial coverage level is smaller than the number of randomaccesses for this coverage level. If so, the method proceeds with stepS403; otherwise the method proceeds with step S408.

At step 408, if the number of PRACH transmissions at the currentcoverage level is equal to max(NumPreambleAttemptCE-r13) (i.e., thenumber of random accesses for the current coverage level) and smallerthan num(RepetitionsPerPreambleAttempt-r13) (i.e., the maximum number ofrandom accesses), the coverage level is increased by one level, thePRACH resource corresponding to the level is obtained, including atime-domain location, a frequency-domain location and a number ofrepetitions, and the step S403 is performed again.

The UE calculates the PRACH sub-carrier ID to be used at the newcoverage level.

The UE transmits PRACH over the re-determined PRACH resource.

For example, the UE can calculate the sub-carrier ID to be used at CEL1based on the indicated Subcarrier indication of NPRACH. The calculationcan include but not limited to: PRACH sub-carrier ID=12+(6-0) mod 12=18.The UE retransmits the PRACH signal, starting from a subframe satisfying

${{\left( {{sf} + \left\lfloor \frac{ns}{2} \right\rfloor} \right){mod}\mspace{14mu} 640} = 64},$using a sub-carrier ID of 18.

The above steps are repeated until the random access succeeds or untilthe maximum number of repetitions min(3+3+3,20)=9 is reached.

After the multi-carrier enhancement technique has been introduced inNB-IoT Rel-14, it is allowed to initiate a random access in anchorcarrier(s) and non-anchor carrier(s). PRACH carriers available forrandom access are configured at each coverage level. For a base stationinitiated random access process, in addition to the above PRACH resourceinformation, the base station indicates a PRACH carrier for a terminalto initiate random access. When the coverage level needs to be increasedafter the terminal has failed to initiate the random access at theinitial coverage level indicated by the base station, the PRACH carrierindicated by the base station may not be included in the set of PRACHresources for the higher level. In this case, the UE would not be ableto initiate a random access or the UE transmits a PRACH signal over thePRACH resource indicated by the base station but the base station wouldnot respond. In either case, the random access will fail, resulting in areduced success rate of the base station initiated random accesses andloss of uplink synchronization of the terminal.

Optionally, the target PRACH resource further includes a target carrierfor transmitting the PRACH signal. The operation of mapping the resourceinformation to the target PRACH resource from resources available at thecurrent coverage level includes: determining the target carrier based ona list of carriers at the current coverage level and a carrier index inthe resource information.

Optionally, the operation of determining the target carrier based on thelist of carriers at the current coverage level and the carrier index inthe resource information includes: determining a list of target carriersat the current coverage level based on a list of non-anchor carriersconfigured by the system and PRACH resource configurations for differentcoverage levels on each carrier, wherein the list of non-anchor carriersstores first non-anchor carriers, a second non-anchor carrier in thelist of target carriers is included in the list of non-anchor carriers,and each carrier in the list of target carriers has an index;calculating a fourth index, ID12, based on a number, Num4, of carriersin the list of target carriers and the carrier index, ID11, asID12=(ID11)mod(Num4), wherein the carrier index ID11 is an index forsearching for the target carrier; and determining a carrier having theindex of ID12 in the list of target carriers as the target carrier.

Each non-anchor carrier is configured by the system with PRACH resourcescorresponding to the respective coverage levels. According to the orderin the list of non-anchor carriers configured by the system, thenon-anchor carriers each configured with the PRACH resource for thecurrent coverage level are organized, in their sequence in the list ofnon-anchor carriers, into the list of target carriers.

Optionally, when the carrier index ID11 is not equal to a predeterminedthreshold, each of the first non-anchor carriers in the list of thenon-anchor carriers has a carrier sequence number, an anchor carrier atthe head of the list of target carriers has a carrier sequence number of0, and the second non-anchor carrier has a same carrier sequence numberas a carrier, from which the second non-anchor carrier originates, inthe list of the non-anchor carriers.

Optionally, when the carrier index ID11 is not equal to a predeterminedthreshold, an anchor carrier at the head of the list of target carriershas a carrier sequence number of 0, and the second non-anchor carrierhas a carrier sequence number that is the same as the index of thesecond non-anchor carrier.

Optionally, when the carrier index ID11 is not equal to a predeterminedthreshold, each of the first non-anchor carriers in the list of thenon-anchor carriers has a carrier sequence number, an anchor carrier atthe end of the list of target carriers has a carrier sequence number of0, and the second non-anchor carrier has a same carrier sequence numberas a carrier, from which the second non-anchor carrier originates, inthe list of the non-anchor carriers.

Optionally, when the carrier index ID11 is equal to a predeterminedthreshold, each of the first non-anchor carriers in the list of thenon-anchor carriers has a carrier sequence number, and, in the list oftarget carriers, the second non-anchor carrier has a same carriersequence number as a carrier, from which the second non-anchor carrieroriginates, in the list of the non-anchor carriers. The list of targetcarriers includes no anchor carrier.

Optionally, when the carrier index ID11 is equal to a predeterminedthreshold, in the list of target carriers, the second non-anchor carrierhas a carrier sequence number that is the same as the index of thesecond non-anchor carrier. The list of target carriers includes noanchor carrier.

Optionally, when the carrier index ID11 is equal to a predeterminedthreshold, an anchor carrier is the target carrier. The predeterminedthreshold indicates the anchor carrier as the target carrier.

The operation of determining the target carrier based on the list ofcarriers at the current coverage level includes: determining a list, A,of available carriers at the current coverage level based on the list ofnon-anchor carriers configured by the system, {non-anchor carrier ID1,non-anchor carrier ID2, . . . }, and PRACH resource configurations forrespective coverage levels on each carrier. The carriers in the list Aare arranged in the order of {anchor carrier, non-anchor carrier n1,non-anchor carrier n2, . . . }. Indices 0, 1, 2, . . . are assigned tothe carriers in the list A in that order. The anchor carrier has acarrier ID of 0. The carrier IDs of non-anchor carrier n1, non-anchorcarrier n2, . . . are carrier IDs of the respective non-anchor carriersin the list of non-anchor carriers configured by the system, in theorder that is the same as, or conforms to, the order in the list ofnon-anchor carriers configured by the system. The fourth index ID12 iscalculated based on the number Num4 of carriers in the list A ofcarriers and the carrier index ID11, as ID12=(ID11)mod(Num4). A carrierhaving the index of ID12 in the list A of carriers is determined as thetarget carrier.

Alternatively, a list, A, of available carriers at the current coveragelevel can be determined based on the list of non-anchor carriersconfigured by the system, {non-anchor carrier ID1, non-anchor carrierID2, . . . }, and PRACH resource configurations for respective coveragelevels on each carrier. The carriers in the list A are arranged in theorder of {anchor carrier, non-anchor carrier n1, non-anchor carrier n2,. . . }. Indices 0, 1, 2, . . . are assigned to the carriers in the listA in that order. The carrier IDs of the anchor carrier, non-anchorcarrier n1, non-anchor carrier n2, . . . are their respective indices.The order of non-anchor carrier n1, non-anchor carrier n2, . . . is thesame as the order in the list of non-anchor carriers configured by thesystem. The fourth index ID12 is calculated based on the number Num4 ofcarriers in the list A of carriers and the carrier index ID11, asID12=(ID11)mod(Num4). A carrier having the index of ID12 in the list Aof carriers is determined as the target carrier.

Alternatively, a list, A, of available carriers at the current coveragelevel can be determined based on the list of non-anchor carriersconfigured by the system, {non-anchor carrier ID1, non-anchor carrierID2, . . . }, and PRACH resource configurations for respective coveragelevels on each carrier. The carriers in the list A are arranged in theorder of {non-anchor carrier n1, non-anchor carrier n2, . . . , anchorcarrier}. Indices 0, 1, 2, . . . are assigned to the carriers in thelist A in that order. The anchor carrier has a carrier ID of 0. Thecarrier IDs of non-anchor carrier n1, non-anchor carrier n2, . . . arecarrier IDs of the respective non-anchor carriers in the list ofnon-anchor carriers configured by the system, in the order that is thesame as, or conforms to, the order in the list of non-anchor carriersconfigured by the system. The fourth index ID is calculated based on thenumber Num4 of carriers in the list A of carriers and the carrier indexID11, as ID12=(ID11)mod(Num4). A carrier having the index of ID12 in thelist A of carriers is determined as the target carrier.

Alternatively, if the carrier index ID11 is a particular value, theanchor carrier is determined as the target sub-carrier. If the carrierindex ID11 is not the particular value, a list, A, of available carriersat the current coverage level can be determined based on the list ofnon-anchor carriers configured by the system, {non-anchor carrier ID1,non-anchor carrier ID2, . . . }, and PRACH resource configurations forrespective coverage levels on each carrier. The carriers in the list Aare arranged in the order of {non-anchor carrier n1, non-anchor carriern2, . . . }. Indices 0, 1, 2, . . . are assigned to the carriers in thelist A in that order. The anchor carrier has a carrier ID of 0. Thecarrier IDs of non-anchor carrier n1, non-anchor carrier n2, . . . arecarrier IDs of the respective non-anchor carriers in the list ofnon-anchor carriers configured by the system, in the order that is thesame as, or conforms to, the order in the list of non-anchor carriersconfigured by the system. The fourth index ID12 is calculated based onthe number Num4 of carriers in the list A of carriers and the carrierindex ID11, as ID12=(ID11)mod(Num4). A carrier having the index of ID12in the list A of carriers is determined as the target carrier.

Alternatively, if the carrier index ID11 is a particular value, theanchor carrier is determined as the target sub-carrier. If the carrierindex ID11 is not the particular value, a list, A, of available carriersat the current coverage level can be determined based on the list ofnon-anchor carriers configured by the system, {non-anchor carrier ID1,non-anchor carrier ID2, . . . }, and PRACH resource configurations forrespective coverage levels on each carrier. The carriers in the list Aare arranged in the order of {anchor carrier, non-anchor carrier n1,non-anchor carrier n2, . . . }. Indices 0, 1, 2, . . . are assigned tothe carriers in the list Ain that order. The carrier IDs of non-anchorcarrier n1, non-anchor carrier n2, . . . are their respective indices.The order of non-anchor carrier n1, non-anchor carrier n2, . . . is thesame as the order in the list of non-anchor carriers configured by thesystem. The fourth index ID12 is calculated based on the number Num4 ofcarriers in the list A of carriers and the carrier index ID11, asID12=(ID11)mod(Num4). A carrier having the index of ID in the list A ofcarriers is determined as the target carrier.

Table 9 shows PRACH information indicated in a PDCCH order according toNB-IoT Rel-14 standard protocol.

TABLE 9 NB-IoT Description Starting number of Coverage level at which UEtransmits NPRACH repetitions PRACH initially, i.e., UE initiates randomaccess over a PRACH resource set corresponding to the coverage levelinitially Subcarrier indication of Frequency-domain sub-carrier ID (NB-NPRACH IoT) for UE to transmit PRACH PRACH carrier Index Carrier indexfor UE to transmit PRACH initially

In the NB-IoT system, for a Rel-13 terminal, it transmits a PRACH signalon an anchor carrier and determines the sub-carrier for transmission ofPRACH. For a Rel-14 terminal, it determines the PRACH carrier fortransmission of a PRACH signal at the current coverage level based onthe “PRACH carrier Index” field indicated by the base station, inaccordance with the following non-limiting schemes.

Scheme 1:

1) All PRACH carriers available for transmitting the PRACH signal at thecurrent coverage level are determined. If there is an anchor carrier, itwill be numbered as 0 and non-anchor carriers will be numberedsequentially, starting from 1, according to their orders in a list ofnon-anchor carriers in a broadcast message. If there is no anchorcarrier, non-anchor carriers will be numbered sequentially, startingfrom 0, according to their orders in a list of non-anchor carriers in abroadcast message. The numbers are denoted as index1, and the maximumnumber+1 is the number, Nprach_carrier, of PRACH carriers available fortransmitting the PRACH signal at the current coverage level. The numberis denoted as index1, and the maximum number is the number,Nprach_carrier, of PRACH carriers available for transmitting the PRACHsignal at the current coverage level.

2) The PRACH carrier to be used at the current coverage level isdetermined as: index2=index1 mod Nprach_carrier.

3) The PRACH carrier having the value of index2 in index1 is determinedas the PRACH carrier to be used by the terminal for transmitting thePRACH signal.

Scheme 2:

1) All PRACH carriers available for transmitting the PRACH signal at thecurrent coverage level are determined. Non-anchor carriers are numberedsequentially, starting from 0, according to their orders in a list ofnon-anchor carriers in a broadcast message. The maximum number isNnon-anchor. If there is an anchor carrier, it will be numbered asNnon-anchor+1. The numbers are denoted as index1, and the maximumnumber+1 is the number, Nprach_carrier, of PRACH carriers available fortransmitting the PRACH signal at the current coverage level.

2) The PRACH carrier to be used at the current coverage level isdetermined as: index2=index1 mod Nprach_carrier.

3) The PRACH carrier having the value of index2 in index1 is determinedas the PRACH carrier to be used by the terminal for transmitting thePRACH signal.

Scheme 3:

1) If PRACH carrier Index indicated by the system is a particular value(e.g., 0 or 15), it indicates that the PRACH signal is to be transmittedon an anchor carrier. The terminal transmits the PRACH signal only onthe anchor carrier.

2) If PRACH carrier Index indicated by the system is not the particularvalue, all non-anchor carriers available for transmitting the PRACHsignal at the current coverage level are determined. Non-anchor carriersare numbered sequentially, starting from 0, according to their orders ina list of non-anchor carriers in a broadcast message. The numbers aredenoted as index1, and the maximum number+1 is the number,Nprach_carrier, of PRACH carriers available for transmitting the PRACHsignal at the current coverage level.

3) The PRACH carrier to be used at the current coverage level isdetermined as: index2=index1 mod Nprach_carrier.

4) The PRACH carrier having the value of index2 in index1 is determinedas the PRACH carrier to be used by the terminal for transmitting thePRACH signal.

Scheme 4:

1) If PRACH carrier Index indicated by the system is a particular valueof 0, it indicates that the PRACH signal is to be transmitted on ananchor carrier. The terminal transmits the PRACH signal only on theanchor carrier.

2) If PRACH carrier Index indicated by the system is not the particularvalue, all non-anchor carriers available for transmitting the PRACHsignal at the current coverage level are determined. The anchor carrieris numbered as 0. The non-anchor carriers are numbered sequentially,starting from 1, according to their orders in a list of non-anchorcarriers in a broadcast message. The numbers are denoted as index1, andthe maximum number is the number, Nprach_carrier, of PRACH carriersavailable for transmitting the PRACH signal at the current coveragelevel.

3) The PRACH carrier to be used at the current coverage level isdetermined as: index2=1+index1 mod Nprach_carrier.

4) The PRACH carrier having the value of index2 in index1 is determinedas the PRACH carrier to be used by the terminal for transmitting thePRACH signal.

An optional embodiment is described below.

An NB-IoT system is configured with three coverage levels, CEL0, CEL1and CEL2, among which CEL0 is the lowest coverage level. An NB-IoT cellis configured with one anchor carrier and three non-anchor carriers. ThePRACH resource information for these five carriers is shown in Table 10,which gives PRACH information configured by means of NB-IoT systembroadcast.

TABLE 10 Threshold PRACH for no. of start PRACH random accesses CoveragePRACH subframe sub- (with a total Carrier ID level period offset carriernumber of 20) Anchor CEL0 640 ms  8 ms  0-11 3 carrier CEL1 640 ms 64 ms12-23 3 CEL2 640 ms 256 ms  24-35 3 Non-anchor CEL0 640 ms  8 ms  0-11 3carrier 1 CEL1 640 ms 64 ms 12-23 3 CEL2 640 ms 256 ms  24-35 3Non-anchor CEL0 640 ms  8 ms  0-11 3 carrier 2 CEL1 640 ms 64 ms 12-23 3Non-anchor CEL0 640 ms  8 ms  0-11 3 carrier 3

An base station transmits a PDCCH order indicating PRACH information asshown in Table 11.

TABLE 11 Description Value Starting number Coverage level at which UECEL0 of NPRACH transmits PRACH initially, i.e., repetitions UE initiatesrandom access over a PRACH resource set corresponding to the coveragelevel initially Subcarrier Frequency-domain sub-carrier 6 indication ofID (NB-IoT) for UE to transmit NPRACH PRACH PRACH carrier Carrier indexfor UE to transmit 3 Index PRACH initially

A Rel-14 UE receives the PDCCH order, obtains an initial coverage levelof CEL0 and PRACH resource information corresponding to CEL0. The PRACHresource information corresponding to CEL0 is shown in Table 12.

TABLE 12 Threshold for no. of PRACH random start PRACH accesses CoveragePRACH subframe sub- (with a total Carrier ID level period offset carriernumber of 20) Anchor CEL0 640 ms 8 ms 0-11 3 carrier Non-anchor CEL0 640ms 8 ms 0-11 3 carrier 1 Non-anchor CEL0 640 ms 8 ms 0-11 3 carrier 2Non-anchor CEL0 640 ms 8 ms 0-11 3 carrier 3

The anchor carrier is numbered as 0. The non-anchor carriers arenumbered, starting from 1, according to their orders in a list ofcarriers configured by system broadcast. The non-anchor carrier 1 isnumbered as 1, the non-anchor carrier 2 is numbered as 2, and thenon-anchor carrier 3 is numbered as 3. A list of the numbers 0-3 isdenoted as index1, i.e., index1=[0,1,2,3], corresponding to {anchorcarrier, non-anchor carrier 1, non-anchor carrier 2, non-anchor carrier3}. Meanwhile, it knows that the number of PRACH carriers at CEL0 is 4.

The UE determines the PRACH carrier index to be used at CEL0 based onthe indicated PRACH carrier Index: index2=PRACH carrier Index mod 4=3mod 4=3.

The carrier having an index of 3 in index1, i.e., the non-anchor carrier3, is determined as the PRACH carrier to be used by the UE fortransmitting the PRACH signal.

On the non-anchor carrier 3, the UE calculates the sub-carrier ID to beused at CEL0 based on the indicated Subcarrier indication of NPRACH,according to, as a non-limiting example:PRACH carrier ID=0+6 mod 12=6.

The UE transmits the PRACH signal, starting from a subframe satisfying

${{\left( {{sf} + \left\lfloor \frac{ns}{2} \right\rfloor} \right){mod}\mspace{14mu} 640} = 8},$using a sub-carrier ID of 6 on the non-anchor carrier 3, where sf is aradio frame number and ns is a slot number.

If the initiated random access process fails and the number of randomaccesses initiated at CEL0 is smaller than 3, the PRACH signal isretransmitted, starting from a subframe satisfying

${{\left( {{sf} + \left\lfloor \frac{ns}{2} \right\rfloor} \right){mod}\mspace{14mu} 640} = 8},$using a sub-carrier ID of 6 on the non-anchor carrier 3.

If the number of random accesses initiated at CEL0 is 3 (which has notreached the total number threshold of 20), the UE increases the coveragelevel to CEL1 and obtains the PRACH resource corresponding to the level,as shown in Table 13.

TABLE 13 Threshold for no. of PRACH random start PRACH accesses CoveragePRACH subframe sub- (with a total Carrier ID level period offset carriernumber of 20) Anchor CEL1 640 ms 64 ms 12-23 3 carrier Non-anchor CEL1640 ms 64 ms 12-23 3 carrier 1 Non-anchor CEL1 640 ms 64 ms 12-23 3carrier 2

The anchor carrier is numbered as 0. The non-anchor carriers arenumbered, starting from 1, according to their orders in a list ofcarriers configured by system broadcast. The non-anchor carrier 1 isnumbered as 1 and the non-anchor carrier 2 is numbered as 2. A list ofthe numbers 0-2 is denoted as index1, i.e., index1=[0,1,2],corresponding to {anchor carrier, non-anchor carrier 1, non-anchorcarrier 2}. Meanwhile, it knows that the number of PRACH carriers atCEL0 is 3.

The UE determines the PRACH carrier index to be used at CEL1 based onthe indicated PRACH carrier Index: index2=PRACH carrier Index mod 3=3mod 3=0.

The carrier having an index of 0 in index1, i.e., the anchor carrier, isdetermined as the PRACH carrier to be used by the UE for transmittingthe PRACH signal.

On the anchor carrier, the UE calculates the sub-carrier ID to be usedat CEL1 based on the indicated Subcarrier indication of NPRACH,according to, as a non-limiting example:PRACH carrier ID=12+6 mod 12=18.

The UE retransmits the PRACH signal, starting from a subframe satisfying

${{\left( {{sf} + \left\lfloor \frac{ns}{2} \right\rfloor} \right){mod}\mspace{14mu} 640} = 8},$using a sub-carrier ID of 18 on the anchor carrier.

The above steps are repeated until the random access succeeds or untilthe maximum number of repetitions min(3+3+3,20)=9 is reached.

A further optional example is provided according to this embodiment.

An NB-IoT system is configured with three coverage levels, CEL0, CEL1and CEL2, among which CEL0 is the lowest coverage level. An NB-IoT cellis configured with one anchor carrier and three non-anchor carriers. ThePRACH resource information for these five carriers is shown in Table 14.

TABLE 14 Threshold for no. of PRACH random start PRACH accesses CoveragePRACH subframe sub- (with a total Carrier ID level period offset carriernumber of 20) Anchor CEL0 640 ms  8 ms  0-11 3 carrier CEL1 640 ms  64ms 12-23 3 CEL2 640 ms 256 ms 24-35 3 Non-anchor CEL0 640 ms  8 ms  0-113 carrier 1 CEL1 640 ms  64 ms 12-23 3 CEL2 640 ms 256 ms 24-35 3Non-anchor CEL0 640 ms  8 ms  0-11 3 carrier 2 CEL1 640 ms  64 ms 12-233 Non-anchor CEL0 640 ms  8 ms  0-11 3 carrier 3

An base station transmits a PDCCH order indicating PRACH information asshown in Table 15. Table 15 shows the PRACH information indicated in thePDCCH order transmitted in NB-IoT.

TABLE 15 Description Value Starting number Coverage level at which UECEL0 of NPRACH transmits PRACH initially, i.e., repetitions UE initiatesrandom access over a PRACH resource set corresponding to the coveragelevel initially Subcarrier Frequency-domain sub-carrier 6 indication ofID (NB-IoT) for UE to transmit NPRACH PRACH PRACH carrier Carrier indexfor UE to transmit 3 Index PRACH initially

A Rel-14 UE receives the PDCCH order. The indicated PRACH carrier Indexis not a particular value (assumed to be 0). Then, the UE obtains aninitial coverage level of CEL0 and PRACH resource information fornon-anchor carriers corresponding to CEL0. The PRACH resourceinformation for non-anchor carriers corresponding to CEL0 is shown inTable 16.

TABLE 16 Threshold for no. of PRACH random start PRACH accesses CoveragePRACH subframe sub- (with a total Carrier ID level period offset carriernumber of 20) Non-anchor CEL0 640 ms 8 ms 0-11 3 carrier 1 Non-anchorCEL0 640 ms 8 ms 0-11 3 carrier 2 Non-anchor CEL0 640 ms 8 ms 0-11 3carrier 3

The non-anchor carriers are numbered, starting from 0, according totheir orders in a list of carriers configured by system broadcast. Thenon-anchor carrier 1 is numbered as 0, the non-anchor carrier 2 isnumbered as 1, and the non-anchor carrier 3 is numbered as 2. A list ofthe numbers 0-2 is denoted as index1, i.e., index1=[0,1,2],corresponding to {non-anchor carrier 1, non-anchor carrier 2, non-anchorcarrier 3}. Meanwhile, it knows that the number of non-anchor PRACHcarriers at CEL0 is 3.

The UE determines the non-anchor PRACH carrier index to be used at CEL0based on the indicated PRACH carrier Index: index2=PRACH carrier Indexmod 3=3 mod 3=0.

The carrier having an index of 0 in index1, i.e., the non-anchor carrier1, is determined as the PRACH carrier to be used by the UE fortransmitting the PRACH signal.

On the non-anchor carrier 1, the UE calculates the sub-carrier ID to beused at CEL0 based on the indicated Subcarrier indication of NPRACH,according to, as a non-limiting example:PRACH carrier ID=0+6 mod 12=6.

The UE transmits the PRACH signal, starting from a subframe satisfying

${{\left( {{sf} + \left\lfloor \frac{ns}{2} \right\rfloor} \right){mod}\mspace{14mu} 640} = 8},$using a sub-carrier ID of 6 on the non-anchor carrier 1, where sf is aradio frame number and ns is a slot number.

If the initiated random access process fails and the number of randomaccesses initiated at CEL0 is smaller than 3, the PRACH signal isretransmitted, starting from a subframe satisfying

${{\left( {{sf} + \left\lfloor \frac{ns}{2} \right\rfloor} \right){mod}\mspace{14mu} 640} = 8},$using a sub-carrier ID of 6 on the non-anchor carrier 1.

If the number of random accesses initiated at CEL0 is 3 (which has notreached the total number threshold of 20), the UE increases the coveragelevel to CEL1 and obtains the non-anchor PRACH resource corresponding tothe level, as shown in Table 17.

TABLE 17 Threshold for no. of PRACH random start PRACH accesses CoveragePRACH subframe sub- (with a total Carrier ID level period offset carriernumber of 20) Non-anchor CEL1 640 ms 64 ms 12-23 3 carrier 1 Non-anchorCEL1 640 ms 64 ms 12-23 3 carrier 2

The non-anchor carriers are numbered, starting from 0, according totheir orders in a list of carriers configured by system broadcast. Thenon-anchor carrier 1 is numbered as 0 and the non-anchor carrier 2 isnumbered as 1. A list of the numbers 0-1 is denoted as index1, i.e.,index1=[0,1], corresponding to {non-anchor carrier 1, non-anchor carrier2}. Meanwhile, it knows that the number of non-anchor PRACH carriers atCEL0 is 2.

The UE determines the PRACH carrier index to be used at CEL1 based onthe indicated PRACH carrier Index: index2=PRACH carrier Index mod 2=3mod 2=1.

The carrier having an index of 1 in index1, i.e., the non-anchor carrier2, is determined as the PRACH carrier to be used by the UE fortransmitting the PRACH signal.

On the non-anchor carrier 2, the UE calculates the sub-carrier ID to beused at CEL1 based on the indicated Subcarrier indication of NPRACH,according to, as a non-limiting example:PRACH carrier ID=12+6 mod 12=18.

The UE retransmits the PRACH signal, starting from a subframe satisfying

${{\left( {{sf} + \left\lfloor \frac{ns}{2} \right\rfloor} \right){mod}\mspace{14mu} 640} = 8},$using a sub-carrier ID of 18 on the non-anchor carrier 2.

The above steps are repeated until the random access succeeds or untilthe maximum number of repetitions min(3+3+3,20)=9 is reached.

A further optional example is provided according to the presentdisclosure.

An NB-IoT system is configured with three coverage levels, CEL0, CEL1and CEL2, among which CEL0 is the lowest coverage level. An NB-IoT cellis configured with one anchor carrier and three non-anchor carriers. ThePRACH resource information for these five carriers is shown in Table 18,which gives PRACH information configured by means of NB-IoT systembroadcast.

TABLE 18 Threshold for no. of PRACH random start PRACH accesses CoveragePRACH subframe sub- (with a total Carrier ID level period offset carriernumber of 20) Anchor CEL0 640 ms  8 ms  0-11 3 carrier CEL1 640 ms  64ms 12-23 3 CEL2 640 ms 256 ms 24-35 3 Non-anchor CEL0 640 ms  8 ms  0-113 carrier 1 CEL1 640 ms  64 ms 12-23 3 CEL2 640 ms 256 ms 24-35 3Non-anchor CEL0 640 ms  8 ms  0-11 3 carrier 2 CEL1 640 ms  64 ms 12-233 Non-anchor CEL0 640 ms  8 ms  0-11 3 carrier 3

An base station transmits a PDCCH order indicating PRACH information asshown in Table 19. Table 19 shows the PRACH information indicated in thePDCCH order transmitted in NB-IoT.

TABLE 19 Description Value Starting number Coverage level at which UECEL0 of NPRACH transmits PRACH initially, i.e., repetitions UE initiatesrandom access over a PRACH resource set corresponding to the coveragelevel initially Subcarrier Frequency-domain sub-carrier 6 indication ofID (NB-IoT) for UE to transmit NPRACH PRACH PRACH carrier Carrier indexfor UE to transmit 3 Index PRACH initially

A Rel-14 UE receives the PDCCH order. The indicated PRACH carrier Indexis not a particular value (assumed to be 0). Then, the UE obtains aninitial coverage level of CEL0 and PRACH resource information fornon-anchor carriers corresponding to CEL0. The PRACH resourceinformation for non-anchor carriers corresponding to CEL0 is shown inTable 20.

TABLE 20 Threshold for no. of PRACH random start PRACH accesses CoveragePRACH subframe sub- (with a total Carrier ID level period offset carriernumber of 20) Non-anchor CEL0 640 ms 8 ms 0-11 3 carrier 1 Non-anchorCEL0 640 ms 8 ms 0-11 3 carrier 2 Non-anchor CEL0 640 ms 8 ms 0-11 3carrier 3

The anchor carrier is numbered as 0. The non-anchor carriers arenumbered, starting from 1, according to their orders in a list ofcarriers configured by system broadcast. The non-anchor carrier 1 isnumbered as 1, the non-anchor carrier 2 is numbered as 2, and thenon-anchor carrier 3 is numbered as 3. A list of the numbers 0-3 isdenoted as index1, i.e., index1=[0,1,2,3], corresponding to {anchorcarrier, non-anchor carrier 1, non-anchor carrier 2, non-anchor carrier3}. Meanwhile, it knows that the number of non-anchor PRACH carriers atCEL0 is 3.

The UE determines the non-anchor PRACH carrier index to be used at CEL0based on the indicated PRACH carrier Index: index2=1+PRACH carrier Indexmod 3=1+3 mod 3=1.

The carrier having an index of 1 in index1, i.e., the non-anchor carrier1, is determined as the PRACH carrier to be used by the UE fortransmitting the PRACH signal.

On the non-anchor carrier 1, the UE calculates the sub-carrier ID to beused at CEL0 based on the indicated Subcarrier indication of NPRACH,according to, as a non-limiting example:

PRACH carrier ID=0+6 mod 12=6.

The UE transmits the PRACH signal, starting from a subframe satisfying

${{\left( {{sf} + \left\lfloor \frac{ns}{2} \right\rfloor} \right){mod}\mspace{14mu} 640} = 8},$

using a sub-carrier ID of 6 on the non-anchor carrier 1, where sf is aradio frame number and ns is a slot number.

If the initiated random access process fails and the number of randomaccesses initiated at CEL0 is smaller than 3, the PRACH signal isretransmitted, starting from a subframe

${{\left( {{sf} + \left\lfloor \frac{ns}{2} \right\rfloor} \right){mod}\mspace{14mu} 640} = 8},$

satisfying using a sub-carrier ID of 6 on the non-anchor carrier 1.

If the number of random accesses initiated at CEL0 is 3 (which has notreached the total number threshold of 20), the UE increases the coveragelevel to CEL1 and obtains the non-anchor PRACH resource corresponding tothe level. Table 21 shows the PRACH resource information for non-anchorcarriers at CEL1.

TABLE 21 Threshold for no. of PRACH random start PRACH accesses CoveragePRACH subframe sub- (with a total Carrier ID level period offset carriernumber of 20) Non-anchor CEL1 640 ms 64 ms 12-23 3 carrier 1 Non-anchorCEL1 640 ms 64 ms 12-23 3 carrier 2

The non-anchor carriers are numbered, starting from 0, according totheir orders in a list of carriers configured by system broadcast asshown in Table 5. The non-anchor carrier 1 is numbered as 0 and thenon-anchor carrier 2 is numbered as 1. A list of the numbers 0-1 isdenoted as index1, i.e., index1=[0,1], corresponding to {non-anchorcarrier 1, non-anchor carrier 2}. Meanwhile, it knows that the number ofnon-anchor PRACH carriers at CEL0 is 2.

The UE determines the PRACH carrier index to be used at CEL1 based onthe indicated PRACH carrier Index: index2=PRACH carrier Index mod 2=3mod 2=1.

The carrier having an index of 1 in index1, i.e., the non-anchor carrier2, is determined as the PRACH carrier to be used by the UE fortransmitting the PRACH signal.

On the non-anchor carrier 2, the UE calculates the sub-carrier ID to beused at CEL1 based on the indicated Subcarrier indication of NPRACH,according to, as a non-limiting example:PRACH carrier ID=12+6 mod 12=18.

The UE retransmits the PRACH signal, starting from a subframe satisfying

${{\left( {{sf} + \left\lfloor \frac{ns}{2} \right\rfloor} \right){mod}\mspace{14mu} 640} = 8},$using a sub-carrier ID of 18 on the non-anchor carrier 2.

The above steps are repeated until the random access succeeds or untilthe maximum number of repetitions min(3+3+3,20)=9 is reached.

A further optional example is provided according to the presentdisclosure.

An NB-IoT system is configured with three coverage levels, CEL0, CEL1and CEL2, among which CEL0 is the lowest coverage level. An NB-IoT cellis configured with one anchor carrier and three non-anchor carriers. ThePRACH resource information for these five carriers is shown in Table 22,which gives PRACH information configured by means of NB-IoT systembroadcast.

TABLE 22 Threshold for no. of PRACH random start PRACH accesses CoveragePRACH subframe sub- (with a total Carrier ID level period offset carriernumber of 20) Anchor CEL0 640 ms  8 ms  0-11 3 carrier CEL1 640 ms  64ms 12-23 3 CEL2 640 ms 256 ms 24-35 3 Non-anchor CEL0 640 ms  8 ms  0-113 carrier 1 CEL1 640 ms  64 ms 12-23 3 CEL2 640 ms 256 ms 24-35 3Non-anchor CEL0 640 ms  8 ms  0-11 3 carrier 2 CEL1 640 ms  64 ms 12-233 Non-anchor CEL0 640 ms  8 ms  0-11 3 carrier 3

An base station transmits a PDCCH order indicating PRACH information asshown in Table 23.

TABLE 23 Description Value Starting number Coverage level at which UECEL0 of NPRACH transmits PRACH initially, i.e., repetitions UE initiatesrandom access over a PRACH resource set corresponding to the coveragelevel initially Subcarrier Frequency-domain sub-carrier 6 indication ofID (NB-IoT) for UE to transmit NPRACH PRACH PRACH carrier Carrier indexfor UE to transmit 3 Index PRACH initially

A Rel-14 UE receives the PDCCH order. The indicated PRACH carrier Indexis a particular value (assumed to be 0). Then, the UE obtains an initialcoverage level of CEL0 and PRACH resource information for the anchorcarrier corresponding to CEL0. The PRACH resource information for theanchor carrier corresponding to CEL0 is shown in Table 24.

TABLE 24 Threshold for no. of PRACH random start PRACH accesses CoveragePRACH subframe sub- (with a total Carrier ID level period offset carriernumber of 20) anchor CEL0 640 ms 8 ms 0-11 3 carrier

On the anchor carrier, the UE calculates the sub-carrier ID to be usedat CEL0 based on the indicated Subcarrier indication of NPRACH,according to, as a non-limiting example:PRACH carrier ID=0+6 mod 12=6.

The UE transmits the PRACH signal, starting from a subframe satisfying

${{\left( {{sf} + \left\lfloor \frac{ns}{2} \right\rfloor} \right){mod}\mspace{14mu} 640} = 8},$using a sub-carrier ID of 6 on the anchor carrier, where sf is a radioframe number and ns is a slot number.

If the initiated random access process fails and the number of randomaccesses initiated at CEL0 is smaller than 3, the PRACH signal isretransmitted, starting from a subframe satisfying

${{\left( {{sf} + \left\lfloor \frac{ns}{2} \right\rfloor} \right){mod}\mspace{14mu} 640} = 8},$using a sub-carrier ID of 6 on the anchor carrier.

If the number of random accesses initiated at CEL0 is 3 (which has notreached the total number threshold of 20), the UE increases the coveragelevel to CEL1 and obtains the non-anchor PRACH resource corresponding tothe level, as shown in Table 25.

TABLE 25 Threshold for no. of PRACH random start PRACH accesses CoveragePRACH subframe sub- (with a total Carrier ID level period offset carriernumber of 20) anchor CEL1 640 ms 64 ms 12-23 3 carrier

On the anchor carrier, the UE calculates the sub-carrier ID to be usedat CEL1 based on the indicated Subcarrier indication of NPRACH,according to, as a non-limiting example:PRACH carrier ID=12+6 mod 12=18.

The UE retransmits the PRACH signal, starting from a subframe satisfying

${{\left( {{sf} + \left\lfloor \frac{ns}{2} \right\rfloor} \right){mod}\mspace{14mu} 640} = 8},$using a sub-carrier ID of 18 on the anchor carrier.

The above steps are repeated until the random access succeeds or untilthe maximum number of repetitions min(3+3+3,20)=9 is reached.

With the description of the above embodiments, it will be apparent tothose skilled in the art that the method according to the aboveembodiments can be realized by means of software plus the necessarygeneral hardware platform, and of course can be implemented viahardware, but in many cases the former is the better implementation.Based on this understanding, the technical solution of the presentdisclosure, in essence or in terms of parts contributing to the priorart, can be embodied in the form of a software product, which is storedin a storage medium (e.g., ROM/RAM, disc, CD-ROM) and includes a numberof instructions for enabling a terminal device (which may be a mobilephone, a computer, a server, or a network device, etc.) to perform themethod described in the various embodiments of the present disclosure.

Embodiment 2

According to an embodiment of the present disclosure, an apparatus forrandom access in a wireless communication system is also provided. Theapparatus can implement the above embodiments and preferably embodimentsand details thereof will be omitted here. As used hereinafter, the term“module” can be software, hardware, or a combination thereof, capable ofperforming a predetermined function. While the apparatuses as describedin the following embodiments are preferably implemented in software, itcan be contemplated that they can also be implemented in hardware or acombination of software and hardware.

FIG. 5 is a schematic diagram showing an apparatus for random access ina wireless communication system according to the present disclosure. Asshown in FIG. 5 , the apparatus can include a receiving unit 51, adetermining unit 52 and a transmitting unit 53.

The receiving unit 51 is configured to receive indication informationtransmitted from a radio base station. The indication informationinstructs a user terminal to initiate a random access.

The determining unit 52 is configured to determine a target PhysicalRandom Access Channel (PRACH) resource from available resources based ona current coverage level and resource information carried in theindication information. The target PRACH resource is used fortransmitting a PRACH signal for initiating the random access. Thecurrent coverage level is a coverage level at which the random access isto be initiated currently.

The transmitting unit 53 is configured to re-determine, in response to afailure to initiate the random access over the target PRACH resource,the target PRACH resource from the available resources based on anupdated coverage level and the resource information, and transmit thePRACH signal over the re-determined target PRACH resource.

With the above embodiments, the receiving unit receives indicationinformation transmitted from a radio base station. The indicationinformation instructs a user terminal to initiate a random access. Thedetermining unit determines a target PRACH resource from availableresources based on a current coverage level and resource informationcarried in the indication information. The target PRACH resource is usedfor transmitting a PRACH signal for initiating the random access. Thetransmitting unit re-determines, in response to a failure to initiatethe random access over the target PRACH resource, the target PRACHresource from the available resources based on an updated coverage leveland the resource information, and transmits the PRACH signal over there-determined target PRACH resource. In this way, it is possible tosolve the technical problem in the related art associated with lowsuccess rate of random accesses initiated by radio base stations,thereby achieving a technical effect of an increased success rate ofrandom accesses.

When the receiving unit receives the indication information transmittedfrom the radio base station, it can receive the indication informationas PDCCH order information.

When the determining unit determines the target PRACH resource fromavailable resources based on the current coverage level and the resourceinformation carried in the indication information, it can map theresource information to the target PRACH resource from resourcesavailable at the current coverage level. In particular, it can determinethe target PRACH resource in a manner corresponding to a system type ofthe radio base station.

When the wireless communication system is an enhanced Machine TypeCommunication (eMTC) system, the target PRACH resource includes a targettime-frequency resource and a target codeword for transmitting the PRACHsignal. The determining unit includes: a second determining moduleconfigured to determine the target time-frequency resource based on anumber of time-frequency resources available at the current coveragelevel and a time-frequency resource index in the resource information;and/or a third determining module configured to determine the targetcodeword based on a start codeword and an end codeword of preambles atthe current coverage level and a preamble index in the resourceinformation.

Optionally, the second determining module is further configured to:calculate a first index, ID2, based on the number, Num1, of theavailable time-frequency resources and the time-frequency resourceindex, ID1, in the resource information according to a predeterminedequation of ID2=(ID1−K1)mod(Num1)+K2 or ID2=(ID1)mod(Num1)+K3, where K1,K2 and K3 are constants; and determine a time-frequency resourcecorresponding to ID2 as the target time-frequency resource.

Optionally, the third determining module is further configured to:calculate a second index, ID6, based on an index, ID3, of the startcodeword, an index, ID4, of the end codeword and the preamble index,ID5, as ID6=ID3+(ID5)mod(ID4−ID3+1), or calculate the second index ID6based on the index ID3 of the start codeword, the index ID4 of the endcodeword and a start codeword index, ID9, corresponding to a coveragelevel indicated in the indication information, asID6=ID3+(ID5−ID9)mod(ID4−ID3+1); and determine a codeword correspondingto ID6 as the target codeword.

When the wireless communication system is a Narrow Band Internet ofThings (NB-IoT) system, the target PRACH resource includes a targetsub-carrier for transmitting the PRACH signal. The determining unitincludes: a fourth determining unit configured to determine the targetsub-carrier based on a number of sub-carriers and an index of a startsub-carrier at the current coverage level and a sub-carrier index oroffset in the resource information.

Optionally, the fourth determining module is further configured to:calculate a third index, ID8, based on the number, Num2, of sub-carriersand the index, ID7, of the start sub-carrier at the current coveragelevel and the sub-carrier offset, Num3, in the resource information, asID8=ID7+(Num3)mod(Num2), or calculate the third index, ID8, based on thenumber Num2 of sub-carriers and the index ID7 of the start sub-carrierat the current coverage level, the sub-carrier index Num3 in theresource information and a start sub-carrier index, ID10, correspondingto a coverage level indicated in the indication information, asID8=ID7+(Num3−ID10)mod(Num2); and determine the sub-carriercorresponding to ID8 as the target sub-carrier.

The above transmitting unit includes: an obtaining module configured toobtain a number of failures to initiate the random access; and a firstdetermining module configured to determine the failure to initiate therandom access over the target PRACH resource when the number of failuresreaches a predetermined value that is a maximum number of transmissionsof the PRACH signal allowable at the current coverage level.

Optionally, the transmitting unit further includes: a transmittingmodule configured to retransmit, after obtaining the number of failuresto initiate the random access, the PRACH signal over the target PRACHresource when the number of failures is smaller than the predeterminedvalue.

It is to be noted that the updated coverage level is one level higherthan the coverage level before updating.

It should be noted that each of the above-described modules can beimplemented by means of software or hardware, and the latter can beimplemented by, but not limited to, the following manner: theabove-mentioned modules can be located at the same processor, or anycombination of the above-mentioned modules can be distributed over aplurality of processors.

Embodiment 3

According to an embodiment of the present disclosure, a storage mediumis also provided. The storage medium can be configured to store programcodes for the following steps.

At S1, indication information transmitted from a radio base station isreceived. The indication information instructs a user terminal toinitiate a random access.

At S2, a target PRACH resource is determined from available resourcesbased on a current coverage level and resource information carried inthe indication information. The target PRACH resource is used fortransmitting a PRACH signal for initiating the random access.

At S3, in response to a failure to initiate the random access over thetarget PRACH resource, the target PRACH resource is re-determined fromthe available resources based on an updated coverage level and theresource information, and the PRACH signal is transmitted over there-determined target PRACH resource.

Alternatively, in the present embodiment, the above-described storagemedium may include, but not limited to, a USB disk, a Read-Only Memory(ROM), a Random Access Memory (RAM), a mobile hard disk, a magnetic Discor CD-ROM and other media that can store program code.

Optionally, in the present embodiment, the processor executes, inaccordance with the program code stored in the storage medium, steps of:receiving indication information transmitted from a radio base station,the indication information instructing a user terminal to initiate arandom access; determining a target Physical Random Access Channel(PRACH) resource from available resources based on a current coveragelevel and resource information carried in the indication information,the target PRACH resource being used for transmitting a PRACH signal forinitiating the random access; and re-determining, in response to afailure to initiate the random access over the target PRACH resource,the target PRACH resource from the available resources based on anupdated coverage level and the resource information, and transmittingthe PRACH signal over the re-determined target PRACH resource.

Optionally, regarding the specific examples in this embodiment,reference can be made to the above embodiments and the examplesdescribed in optional embodiments, and the details thereof will beomitted here.

It will be apparent to those skilled in the art that the above-describedmodules or steps of the present disclosure may be implemented by meansof a general purpose computing device, and may be placed at a singlecomputing device or distributed over a network of multiple computingdevices. Optionally, they may be implemented by means of program codeexecutable by the computing device, so that they may be stored in astorage device and executed by a computing device, and in some cases,they may be performed in a different order from the steps shown ordescribed here, or they may be made separately into individualintegrated circuit modules, or may be implemented by making multiplemodules or steps therein into a single integrated circuit module. Thus,the present disclosure is not limited to any particular hardware,software, and the combination thereof.

The foregoing is merely illustrative of the preferred embodiments of thepresent disclosure and is not intended to limit the present disclosure.Various changes and modifications may be made by those skilled in theart. Any modifications, equivalent substitutions, improvements, and thelike within the spirit and principles of the disclosure are intended tobe included within the scope of the present disclosure.

What is claimed is:
 1. A method for wireless communication, comprising:selecting, by a terminal device, a target carrier for a random-accessprocedure at a current coverage level from a list of carriers configuredby a base station, wherein the list of carriers comprises a list ofnon-anchor carriers each configured with a random-access resource for arespective coverage level among a set of different coverage levels,wherein each of the list of carriers is associated with a carrier index,and wherein the target carrier is determined based on a carrier indexthat is equal to (a carrier indication value for selecting the targetcarrier) modulo (a number of carriers in the list of carriers); andperforming, by the terminal device, the random-access procedure with thebase station based on a random-access resource of the target carrier atthe current coverage level.
 2. The method of claim 1, wherein carriersin the list of carriers are ordered as a sequence according to a carrierindex.
 3. The method of claim 1, wherein the list of carriers furthercomprises an anchor carrier.
 4. The method of claim 3, wherein theanchor carrier has a carrier index that is equal to
 0. 5. The method ofclaim 3, wherein, in case the carrier indication value for selecting thetarget carrier is equal to a predetermined threshold, the target carrieris determined to be the anchor carrier.
 6. A method for wirelesscommunication, comprising: configuring, by a base station, a list ofcarriers for selecting a target carrier for a random-access procedure ata current coverage level, wherein the list of carriers comprises a listof non-anchor carriers each configured with a random-access resource fora respective coverage level among a set of different coverage levels,wherein each of the list of carriers is associated with a carrier index,and wherein the target carrier is determined based on a carrier indexthat is equal to (a carrier indication value for selecting the targetcarrier) modulo (a number of carriers in the list of carriers); andperforming the random-access procedure based on a random-access resourceof the target carrier at the current coverage level.
 7. The method ofclaim 6, wherein carriers in the list of carriers are ordered as asequence according to a carrier index.
 8. The method of claim 6, whereinthe list of carriers further comprises an anchor carrier.
 9. The methodof claim 8, wherein the anchor carrier has a carrier index that is equalto
 0. 10. The method of claim 8, wherein, in case the carrier indicationvalue for selecting the target carrier is equal to a predeterminedthreshold, the target carrier is determined to be the anchor carrier.11. An apparatus for wireless communication, comprising a processor thatis configured to: select a target carrier for a random-access procedureat a current coverage level from a list of carriers configured by a basestation, wherein the list of carriers comprises a list of non-anchorcarriers each configured with a random-access resource for a respectivecoverage level among a set of different coverage levels, wherein each ofthe list of carriers is associated with a carrier index, and wherein thetarget carrier is determined based on a carrier index that is equal to(a carrier indication value for selecting the target carrier) modulo (anumber of carriers in the list of carriers); and perform therandom-access procedure with the base station based on a random-accessresource of the target carrier at the current coverage level.
 12. Theapparatus of claim 11, wherein carriers in the list of carriers areordered as a sequence according to a carrier index.
 13. The apparatus ofclaim 11, wherein the list of carriers further comprises an anchorcarrier.
 14. The apparatus of claim 13, wherein the anchor carrier has acarrier index that is equal to
 0. 15. The apparatus of claim 13,wherein, in case the carrier indication value for selecting the targetcarrier is equal to a predetermined threshold, the target carrier isdetermined to be the anchor carrier.
 16. An apparatus for wirelesscommunication, comprising a processor that is configured to: configure alist of carriers for selecting a target carrier for a random-accessprocedure at a current coverage level, wherein the list of carrierscomprises a list of non-anchor carriers each configured with arandom-access resource for a respective coverage level among a set ofdifferent coverage levels, wherein each of the list of carriers isassociated with a carrier index, and wherein the target carrier isdetermined based on a carrier index that is equal to (a carrierindication value for selecting the target carrier) modulo (a number ofcarriers in the list of carriers); and perform the random-accessprocedure based on a random-access resource of the target carrier at thecurrent coverage level.
 17. The apparatus of claim 16, wherein carriersin the list of carriers are ordered as a sequence according to a carrierindex.
 18. The apparatus of claim 16, wherein the list of carriersfurther comprises an anchor carrier.
 19. The apparatus of claim 18,wherein the anchor carrier has a carrier index that is equal to
 0. 20.The apparatus of claim 18, wherein, in case the carrier indication valuefor selecting the target carrier is equal to a predetermined threshold,the target carrier is determined to be the anchor carrier.
 21. A systemfor wireless communication, comprising: a base station configured toconfigure a list of carriers, wherein the list of carriers comprises alist of non-anchor carriers each configured with a random-accessresource for a respective coverage level among a set of differentcoverage levels, and wherein each of the list of carriers is associatedwith a carrier index, and a terminal device configured to select atarget carrier from the list of carriers for a random-access procedureat a current coverage level, wherein the target carrier is determinedbased on a carrier index that is equal to (a carrier indication valuefor selecting the target carrier) modulo (a number of carriers in thelist of carriers), wherein the base station and the terminal device areconfigured to perform the random-access procedure based on arandom-access resource of the target carrier at the current coveragelevel.